Abstract
Progressive metamorphism of impure dolomitic limestone in the 1.5 to 2.5 km wide contact aureole surrounding the northernmost portion of the boulder batholith has resulted in a consistent sequence of uniformly distributed zones of low-variance mineral parageneses separated by abrupt and distinctive isograds. In silica-undersaturated, aluminous marbles, the following mineral assemblages occur, in order of increasing grade: calcite-dolomite-calcic amphibole-chlorite, calcite-dolomite-calcic amphibole-chlorite-spinel, calcite-dolomite-calcic amphibole-chlorite-olivine-spinel, calcite-dolomite-chlorite-olivine-spinel, calcite-dolomite-olivine-spinel. The spatial distribution of parageneses and the occurrence of low-variance parageneses indicate buffering of the pore fluid composition by the local mineral assemblages. The observed sequence of mineral reactions and the spacing of isograds is in good agreement with experimental and calculated equilibria in terms of P-T-X CO 2and temperatures of equilibration inferred from calcite-dolomite geothermometry, which range from 435 to 607 °C across the aureole.
Microprobe analyses of coexisting minerals indicate attainment of exchange equilibrium. Calcic amphibole and chlorite coexisting with calcite and dolomite become progressively more aluminous with increasing grade; calcic amphibole changes rapidly from Al-poor tremolite to pargasite, while AlIV in Cte increases from 2.0 to 2.3 atoms per 8 tetrahedral sites. Observed low-variance assemblages fix the activities of calcic amphibole and chlorite end-member components as a function of P and T, and hence the systematic compositional variation in these phases is not an independent variable, but is controlled by the local mineral assemblage.
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References
Bierwagen, E.: Geology of the Black Mountain area, Lewis and Clark, and Powell Counties, Montana. Ph. D. Thesis, Princeton Univ. (1964)
Burnham, C.W., Holloway, J.R., Davis, N.F.: Thermodynamic properties of water to 1000 °C and 10,000 bars. Geol. Soc. Am. Spec. Papers 132, 96 (1969)
Fawcett, J.J., Yoder, H.S.: Phase relations of chlorites in the system MgO-Al2O3-SiO2-H2O. Am. Mineralogist 51, 353–380 (1966)
Goldsmith, J.R., Newton, R.C.: P-T-X relations in the system CaCO2-MgCO3 at high temperatures and pressures. Am. J. Sci. 267-A, 160–190 (1969)
Graf, D.L., Goldsmith, J.R.: Dolomite-magnesian calcite relations at elevated temperatures and CO2 pressures. Geochim. Cosmochim. Acta 7, 109–128 (1965)
Greenwood, H.J.: Buffering of pore fluids by metamorphic reactions. Am. J. Sci. 275, 573–594 (1975)
Hutcheon, I., Moore, J.M.: The tremolite isograd near Marble Lake, Ontario. Canad. J. Earth Sci. 10, 936–947 (1973)
Jaeger, J.C.: The temperature in the neighborhood of a cooling intrusive sheet. Am. Jour. Sci. 255, 206–318 (1957)
Knopf, A.: The Boulder bathylith of Montana. Am. J. Sci. 255, 81–103 (1957)
Knopf, A.: Geology of the northern part of the Boulder bathylith and adjacent area, Lewis and Clark and Jefferson Counties, Montana. U.S.G.S. Misc. Geol. Invest. Map I-381 (1963)
Misch, P., Rice, J.M.: Miscibility of tremolite and hornblende in progressive Skagit Metamorphic Suite, North Cascades, Washington. J. Petrol. 16, 1–21 (1975)
Moore, J.N., Kerrick, D.M.: Equilibria in siliceous dolomites of the Alta aureole, Utah. Am. J. Sci. 276, 502–524 (1976)
Rice, J.M.: Phase equilibria and chemistry of metamorphosed impure dolomite, Ross Lake, Washington (abs.). Geol. Soc. Am. Abst. with Progr. 5, 96 (1973)
Rice, J.M.: Progressive metamorphism of impure dolomite and mineral equilibria in the system CaO-MgO-SiO2-K2O-Al2O3-CO2-H2O. Part I: The Marysville aureole, Montana, Part II: The northern portion of the Boulder batholith, Montana. Ph. D. Thesis, University of Washington (1975)
Rice, J.M.: Progressive metamorphism of impure dolomitic limestone in the Marysville aureole, Montana. Am. J. Sci. (in press)
Robie, R.A., Waldbaum, D.R.: Thermodynamic properties of minerals and related substances at 298.15° K (25° C) and one atmosphere (1.013 bars) pressure and at higher temperatures. U.S. Geol. Surv. Bull. 1259, 256 (1968)
Sheppard, M.F.P., Schwarcz, H.P.: Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite. Contrib. Mineral. Petrol. 26, 161–198 (1970)
Skippen, G.B.: Experimental data for reactions in siliceous marbles. J. Geol. 79, 451–481 (1971)
Skippen, G.B.: An experimental model for low pressure metamorphism of siliceous dolomitic marble. Am. J. Sci. 274, 487–509 (1974)
Skippen, G.B, Yzerdraat, W.: XCDFOR: A FORTRAN IV Program for calculating equilibria on T-X CO 2sections. Ottawa, Canada, Carleton Univ. Geol. Pap. 1970–73 (1970)
Trommsdorff, V.: Progressive metamorphose Keiseliger Karbonatgesteine in den Zentralalpen zwischen Bernina und Simplon. Schweiz. Mineral. Petrog. Mitt. 46, 431–447 (1966)
Trommsdorff, V.: Change in T-X during metamorphism of siliceous dolomitic rocks of the Central Alps. Schweiz. Mineral. Petrog. Mitt. 52, 1–4 (1972)
Zen, E-an: Gibbs free energy, enthalpy, and entropy of ten rock-forming minerals: calculations, discrepancies, implications. Am. Mineralogist 57, 524–553 (1972)
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Rice, J.M. Contact metamorphism of impure dolomitic limestone in the Boulder Aureole, Montana. Contrib. Mineral. Petrol. 59, 237–259 (1977). https://doi.org/10.1007/BF00374555
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DOI: https://doi.org/10.1007/BF00374555